Field of Invention
The present disclosure relates to power electronic technology. More particularly, the present disclosure relates to a power circuit and a converter structure capable of two-way energy transmission.
Description of Related Art
With the ever-increasing seriousness of energy-related problems in recent times, the development of new ways in which to generate energy (e.g., wind energy, solar energy, etc.) is attracting the attention of those involved in research and development in various areas. In a power generation system, an AC (alternating current) power outputted by a power generator generally has a frequency and amplitude different from those of an AC electric grid. Thus, the AC power outputted by the power generator is often converted into a DC (direct current) through a converter, and subsequently the DC power is inverted into another AC power which is consistent with the AC electric grid in frequency and phase, so as to connect to the electric grid. In the process of converting the AC power into the DC power and subsequently inverting the DC current into another AC current, electronic components (e.g., power switching valves) are controlled by PWM (pulse-width modulation), and the AC-DC conversion and DC-AC conversion are realized by turning on or off these power switching valves.
In a wind power generation system, for example, a traditional wind power converter generally has two forms, one is a symmetric structure in which the generator-side converter and the grid-side converter both have two levels, and the other is a symmetric structure in which the generator-side converter and the grid-side converter both have three levels. That is, the level number, the switching valve type and the circuit connection of the generator-side converter are the same as those of the grid-side converter. However, although the control scheme of the two-level symmetric structure is simple, when the generator-side converter and the grid-side converter both operate at a lower switching frequency, much sub-harmonic current exists in the output current of the grid-side converter, so that the harmonic content current entering into the electric grid is large. Even when an LCL (inductor-capacitor-inductor) filter structure is used for filtering, the system design becomes complex and even causes more serious harmonic oscillation condition. Furthermore, although the control scheme of the three-level symmetric structure can greatly solve the problem of harmonic current and suppressing EMI (electromagnetic interference), the number of used power components (e.g., insulated gate bipolar transistors) in the main circuit is at least two times larger than that of the two-level structure, so that the construction cost of the system is higher.
In view of the above, many in the industry are endeavoring to find ways in which to design a novel converter structure, which can greatly suppress harmonic current and EMI, reduce the cost of the electric components of a product, reduce the total loss of a wind power generation system and increase energy conversion efficiency.